A tensioner presses, with a specified force, a timing chain or a timing belt that is used for example, for an automobile engine. The tensioner functions to keep the tension of the chain or belt constant and thereby to prevent the chain or belt from becoming elongated or slack.
FIG. 7 shows the condition when a tensioner 100 is mounted to an engine body 200 of an automobile. A pair of cam sprockets 210, 210 and a crank sprocket 220 are arranged inside the engine body 200, and a timing chain 230 is hooked across these sprockets 210 and 220 in an endless manner. A chain guide 240 is swingably arranged along a portion of the movement path of the timing chain 230, and the timing chain 230 slides on the chain guide 240. A mounting surface 250 is formed on the engine body 200, and the tensioner 100 is inserted through the mounting hole 260 and is fixed on the mounting surface 250 by bolts 270. Also, lubricating oil (not shown) is filled inside the engine body 200.
A prior-art tensioner includes (1) a columnar moving member that is movable toward a moving chain, (2) a case that includes a slide hole through which the moving member is inserted so as to move freely and be concentrically arranged with the moving member, (3) a propelling spring that presses the moving member against the case, (4) a holder member that is fitted with the moving member in the concave part that is coaxially formed on the case's tip opening side of the slide hole, displaced in the axial direction of the drive member, and provided concentrically with the concave part, (5) a holder spring that presses the holder member in the moving member's moving direction, (6) multiple locking pieces that slide on an inclined cam and are meshed with multiple locking teeth formed on the outside of the drive member, (7) a cam-introducing ring that is fitted with the drive member in the slide hole and restrains the multiple locking pieces from becoming unengaged from the multiple locking teeth, and (8) an enclosing plate that inserts the drive member, in such a way that the drive member can move forward or backward, and movably encloses the holder spring, the holder member, the locking pieces, and the cam-introducing ring—all of which are sequentially arranged in the concave part of the slide hole. In this tensioner, when the chain extends during the operation of the engine, the drive member sequentially moves forward one tooth at a time, thereby maintaining proper chain tension (for example, see Patent Document 1).
In the above-mentioned tensioner, the drive member is pressed forward by the propelling spring, and the diameter of the locking pieces is enlarged. The locking pieces can move forward while climbing over the locking teeth of the drive member. Also, the locking pieces can be pressed backwards toward the cam inclined face, which reduces the diameter of the locking pieces. The locking pieces are then meshed with the locking teeth of the drive member, restrained from moving backward, and locked.
When mounted to an engine body, the moving member moves forward to a position where a proper chain tension can be maintained, and is prevented from being returned excessively from that forward position while receiving vibrations from the chain guide. If an excessive load is applied, the holder member moves backward, and the holder spring is flexed so as to keep proper chain tension. Also, if the chain is elongated due to use over a long period of time, the moving member moves forward appropriately as the chain guide moves forward, so that proper chain tension can be maintained.
FIG. 8(a) is a vertical cross-sectional view of a prior-art tensioner that has the same structure as the tensioner of the above-mentioned Patent Document 1. FIG. 8(b) is a side view of FIG. 8(a). FIG. 9(a) shows how the moving member of the tensioner in FIG. 8 completely engages with the locking pieces. FIG. 9(b) is a cross-sectional view along line D-D of FIG. 9(a), FIG. 10 (a) is a drawing that shows an enlarged view of the locking pieces shown in FIG. 9 when the moving member advances. FIG. 10 (b) is a cross-sectional view along line E-E of FIG. 10 (a). In these figures, the reference character 310 represents the moving member, 320 represents locking pieces, 330 represents the holder member, 340 represents the propelling spring, 350 represents the pressing spring that presses the locking pieces 320 to the inclined cam face 330a of the holder member 330 and biases the locking pieces in the diameter-reduced direction so that the locking pieces become engaged with the moving member 310, 360 represents the holder spring, and 370 represents the case.